The respiratory rates of various tissues and of an intact animal in Actinia equina L., Nereis diversicolor (O. F. Muller), Mytilus galloprovincialis Lam., Rapana thomasiana Crosse, Carcinus maenas L., Xanto hydrophilus Herbst. were studied, and allometric growth of these species was investigated. A significant correlation was observed between the rate of tissue respiration and the weight of body and tissue. The changes in the rate of tissue respiration can be described by exponential equations, the coefficients of equations being different for different tissues. The changes in the summed respiration of all tissues in an animal with the increase of the body weight are similar qualitatively to the changes of the respiration in the intact animal. It was shown that the decrease of metabolic rate per unit weight in an intact animal with an increase of body weight depend on the decrease of the rate of tissue respiration, the character of the relative growth of the tissue and the nervous and humoral regulation. The relative importance of these factors is unequal for the animals of different systematic groups. The role of the allometric growth and the nervous and hymoral regulation increases with the rise in the organisation of animals.